The present invention is generally directed to printing apparatus, receiver sheets, and finishing processes for such receiver sheets.
Among the technologies available for applying an image to a medium, such as paper, are xerography and direct marking. Common forms of direct marking include ink pen and ink jet marking technologies.
Xerographic printing typically uses a dry toner and produces on a print medium a clear, durable image. However, those familiar with xerography will also recognize that the hardware required for xerographically printing images, and particularly for printing images in multiple colors, may be somewhat complex.
In conventional xerography, electrostatic latent images are formed on a xerographic surface by first uniformly charging a charge retentive surface such as a photoreceptor. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to the original image. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation. This charged pattern is made visible by developing it with toner. Such development includes passing the photoreceptor past one or more developer housings. Color xerographic printing commonly requires multiple developers, generally three color developers (yellow, cyan, and magenta) plus a black developer. The developed image is then fixed to the imaging surface, or is transferred to a receiving medium such as paper, to which it is fixed by suitable fusing techniques.
Direct marking technologies, and in particular ink jet printing, have emerged as printing alternatives that incorporate relatively simpler hardware requirements. In direct marking technologies, ink in the desired image is applied directly to the print medium. Various techniques of direct marking are well understood in the art. For example, the image may be applied by direct contact between a pen and the medium. Alternatively, ink jet recording techniques eject droplets of ink from a printhead onto the medium. Such ink jet techniques may include thermal ink jets, acoustic ink jet, piezo-electric ink jet printing, and others.
However, images produced with the inks used in ink jet marking technologies, and particularly in thermal ink jet marking technologies, do not always exhibit the same level of permanence as xerographically produced images. Typical dye-based and certain pigment based ink jet inks suffer from deficiencies, for example, in water fastness, smear resistance, light-fastness, gloss uniformity, and other appearance properties, after being printed on various substrates. Pigment based inks can provide an image, on a wide variety of substrates, having high optical density with high water fastness, smear resistance and light-fastness, and therefore pigment based are generally preferred to dye-based formulations for archival properties. Dye base ink materials, on the other hand, often more applicable in direct marking technologies and can also exhibit improved colorant properties. Nevertheless, the dye and or pigment based ink images are susceptible to print quality defects and to variability and idiosyncrasies associated with the receiver substrate media, such as, smearing. The images typically remain highly vulnerable to environmental image deterioration.
Xerographically produced images on receiver sheets and similar substrates can also benefit from improvements in their stability, permanence, and resistance to abrasion and adverse environmental effects.
In U.S. Pat. No. 5,847,738, issued Dec. 8, 1998, to Tutt, et al., there is disclosed a process of forming an overcoat on a printed image to provide improved stability comprising: a) applying an image layer on a substrate using a liquid ink to form an imaged element; b) either charging the imaged element to a given polarity or applying a voltage across the surface of the element which is attracted to a conductive surface behind the element; c) applying transparent, charged particles to the element which causes them to be electrostatically attracted to the surface of the image layer; and d) heat-fusing the particles to obtain a protective overcoat of the image layer.
In U.S. Pat. No. 5,612,777, issued Mar. 18, 1997, to Malhotra, there is disclosed an apparatus and method for creating color images which are coated with a composition including a lightfastness inducing material and a hydrophobic polymeric binder which protects the images from rough handling and degradation from exposure to UV radiation.
However, the prior art does not provide for a stand-alone finishing method or apparatus that employs contact development of an image bearing receiver sheet or similar substrate to provide a transparent overcoat on at least one surface of the receiver sheet. Thus, there remains a need for improved image quality and image stability of an image on an image-bearing receiver sheet that has been produced by a variety of printing devices and processes. These and other improvements are accomplished in embodiments of the present invention and as illustrated herein.